Spar systems are used in deep seas of at least about 500 meters depth and usually more, to produce hydrocarbons from undersea wells, as well as to drill the wells and store produced oil. Such systems have a tall and narrow caisson extending down from the sea surface by perhaps one or two hundred meters and riser pipes that extend down from the lower portion of the caisson to the seafloor. Taut mooring lines extend at an incline from the caisson to anchors at the seafloor to limit drift. The tall and narrow caisson is subject to only moderate forces from winds, currents, and waves that cause it to drift from a quiescent position wherein it lies directly over the lower ends of the riser pipes.
Although caisson drift is limited, it still can be substantial in severe weather. When the caisson drifts, its axis remains largely vertical due to ballast at its bottom and buoyancy at its top, and the upper portions of the riser pipes which lie within the caisson also extend vertically. As a result, when the caisson drifts so the lower portions of the riser pipes extend at an incline while upper portions extend vertically, the riser pipes undergo a bend within a height if a few meters at the lower portion of the caisson. Such bending about a relatively small radius of curvature, can reduce the lives of the riser pipes. A system that minimized bending of riser pipes at the bottom of the caisson, when the caisson drifts, would be of value.